Lettermail is fine for thin, light books. You can't ship anything over 500gr or 2cm by LetterMail.
Lettermail is called LetterPost for USA and overseas. LightPacket is the same service, but can be printed from Paypal.
There is no PP discount on the service and no tracking.
After that you are into parcel services and there are dozens of those.
Do NOT use couriers. They are more expensive and no faster than the equivalent Canada Post service. If your customer is not in a fair sized city, the courier will subcontract delivery to Canada Post anyway.
Go to https://www.canadapost.ca/cpotools/apps/far/business/findARate?execution=e1s1 and play with various dimensions and destinations.
For USA I use 90210 as the zipcode.
Use metric. Canada Post went metric in 1974 and everything else is guesswork.
Economy Shipping is a generic term. It just means the seller will probably use a cheap slow service, like Regular* Parcel.
But keep 2cm /500 grams firmly in mind. Nothing larger can go letter rate.
*Which isn't always the cheapest. Like I say, play around on the website.